Layout design data for an integrated circuit are usually checked with design rule checking (DRC) before tape-out to verify manufacturability. After tape-out, however, the layout design data are changed through the mask data preparation (MDP) process and the derived mask data are not identical to the original layout design data. The MDP process contains many types of data conversion and modification such as fracturing, rotation, mirroring, and magnification. During these data conversions, it is quite possible that some errors might be generated, resulting in a defective photomask (or mask). To address this problem, mask rule checking (MRC) has become an important step in a MDP process. Moreover, mask rule checking has propagated up the design flow. Some limited set of mask rules are often embedded into the optical proximity correction step as a convergence constraint. Mask rule checking is also combined with design rule checking to check the layout data that have been processed by resolution enhancement techniques (including OPC).
Mask manufacturing rules (or mask rules) are based on negotiations with mask suppliers and strike a balance between the needs of the patterning technology in the wafer fabrication facility and the manufacturing capability of the mask shop. These rules are usually determined from assumed or experimentally acquired mask-manufacturing limits. It may take dozens of rules to cover mask writing, inspection, data fracture and other critical process steps to ensure the best possible manufacturability, cost and cycle time advantages. FIG. 3 employs a set of geometric shapes to illustrate some mask manufacturing rules. A narrow line space (B) below the resolution capability of a photomask manufacturing process will easily cause a defect of short circuit. Problems may also occur if two corners are too close together (C, D). Detailed discussion can be found in an article by Mason et al, “Mask design rules (45 nm)—time for standardization,” Proceedings of SPIE, Vol. 5992. Similar examples of mask manufacturing rules are also discussed in Kato et al., “Advanced mask rule check (MRC) tool,” Proceedings of SPIE, Vol. 6283 and Gladhill et al., “Advanced manufacturing rules check (MRC) for fully automated assessment of complex reticle designs,” Proceedings of SPIE, Vol. 5992.
These conventional mask manufacturing rules comprise space check, width check, point distance check and perhaps acute angle check. With photolithography being pushed to fabricate deep-subwavelength devices and mask patterns becoming more complex than conventional Manhattan shapes, however, these simple checks may not be sufficient. Multi-patterning and other techniques needed to extend the 193 immersion capabilities usually depend on a large amount of decoration with optical proximity correction (OPC) shapes. Unlike simple orthogonal SRAFs, the SRAFs for 22 nm/20 nm technology nodes tend to be blobs or curvilinear lines. In the contact and via layers, and particularly for isolated features, extensive use of sub-resolution assist features (SRAFs) is needed to produce the required process window. Another source of complex mask patterns is inverse lithography. Masks computed through use of inverse lithography are known to provide significantly better lithographical performance even than conventional model-based OPC. Such masks, however, generally contain patterns with smaller segments and curved shapes. To efficiently deal with complex patterns, new mask rule checks need to be developed.